Minute leak rates of a container can be reliably determined by the vacuum method. The smaller the leak rate, the higher the demands regarding purity and final vacuum. In local leak detection, a vacuum pump evacuates the container until the test pressure required for the leak detector is reached. Suspected leaks are then sprayed with a fine test gas jet from outside. Test gas entering the container is pumped off by the vacuum pump and detected by a mass spectrometer.
A test gas frequently used for leak detectors is helium. There is a problem with the restricted selectivity of the mass spectrometer. Water precipitates both on the outside and on the inside of containers used for leakage measuring. The H2 component of water also includes parts whose presence susceptibly interfere with the measuring of helium. At the beginning of the pumping, the amount signal, which is supposed to represent only the amount of test gas, is superposed by a noise value generated by the presence of water or other contaminations. The noise value fades with increasing pumping time to asymptotically approximate a horizontal line. However, this line never reaches a value of zero, since an absolute vacuum can not be achieved, just as well as an absolute absence of leakage can not be achieved. It is thus a matter of the respective application which portion of the fading volume flow characteristic is selected for leakage measuring.
Since the curve of the volume signal fades as the pumping time increases, the sudden occurrence of test gas that causes a rise in the curve of the volume signal is superimposed by the fading background signal. When the background signal decreases to a greater extent than the detection signal rises, no detection signal is determined at all. The corresponding leak is not detected by the leak detector; it remains invisible.
It has been suggested to provide a leak detector with a zero function. Here, the device is equipped with a zero key that may be pressed by the user to subtract the previous signal from the current signal. Thus, the signal amplitude is set to zero. If then the still fading background signal decreases to a larger extent than the volume signal rises due to the leak, a negative signal results in which the signal rise caused by the leak is neither detectable nor measurable.
In the commonly used methods, the user can press the zero key at any time, the background signal being set to zero. As a consequence, a seemingly low leak rate is indicated, whereas the actual leak rate is higher. Such a mode of operation may have fatal consequences. It is important that no leak rate remains unnoticed. On the other hand, one leak rate indicated too much is less problematic.